1. Field of the Invention
The invention relates to a transdermal therapeutic system for delivering active substances to the human body via the skin.
2. Description of the Relted Art
Transdermal therapeutic systems (TTS) have already been introduced on the market for the therapeutic therapy of a number of illnesses.
A disadvantage of the systems according to the prior art is the insufficient skin permeability of many active substances, which permeability cannot be improved above a certain limit, the so-called "saturation flow", even through numerous galenic measures taken in respect of the TTS design (use of multi-layered systems, use of control membranes, variation of the active substance concentration, modification of the base polymers, etc.). The statement that the transdermal flow of an active substance from the solid, finely dispersed phase can in principle not be increased any further, even despite the use of more strongly dissolving vehicles, can already be found in the works of Higuchi, which are pathbreaking to this day (e.g. T. Higuchi: Physical Chemical Analysis of percutaneous absorptions process from creams and ointments. J. Soc. Cosmetic Chem, 11, S. 85-97 (1960).
With a great number of active substances, however, there is the possibility of adding a penetration enhancer, a so-called "enhancer", to the TTS during its manufacture. Generally, these are liquid or volatile additives improving the absorption properties of the human skin, thus enabling a sufficiently high absorption of the active substance from a relatively small TTS area. However, highly volatile enhancers, such as, for example, ethanol, which is used for the active substance estradiol, present problems due to the strong softening of the adhesive layers of the TTS, thus necessitating further space-consuming compartments in the system, which renders the TTS unacceptably thick or its surface area unacceptably large. Moreover, each additional non-polymeric additive involves a risk of incompatibility reactions on the skin, possibly also of sensitisation. Through adding certain less volatile, but mostly also less active, enhancers (e.g. glycerol esters, cyclic amides, eucalyptol) it is possible to prepare matrix systems which contain the active substance and the absorption-enhancing components in one or more monolithic layers. According to the prior art it is, however, not possible to achieve a satisfactory therapy with such TTS if the enhancers have poor skin compatibility or if the systems, due to the flux through the skin being still too low, require unacceptably large surface areas.
Another possibility of increasing the active substance flow through the skin is to dissolve a greater amount of the active substance in a molecular-disperse manner than corresponds to the saturation solubility. With such oversaturation of these systems, the rate of permeation through the skin increases to the same extent. Since such states are thermodynamically unstable, it is difficult to provide such administration forms in a form which can be stored; a recrystallisation process of active substance particles takes place whose onset and duration cannot be foreseen. This recrystallisation process results in the flow rate through the skin gradually falling to the saturation flow level, thus causing a loss of a large part of the initially present therapeutic activity, depending on the initial concentration.
In a great number of cases where such recrystallisation occurred it was observed that crystallisations have their origin not in the components of the TTS but in the abherent layer.